1. Field of the Invention
The invention relates to defoamer formulations comprising (A) siloxane-based antifoams and (B) silicone polyethers, and also to their use for defoaming aqueous media, particularly those arising in pulp production.
2. Background Art
Siloxane-based defoamers are prepared, for example, according to DD-A 056 762 by heating hydrophilic silica in polydimethylsiloxanes. This process is very elaborate, and yet the activity of the defoamers prepared in this way is unsatisfactory. The dispersion of hydrophobicized silica in a polydimethylsiloxane, in accordance for example, with DE-A 29 25 722, is a more rational process, but the activity of the resulting defoamers is likewise deserving of improvement.
Likewise known is the use of modified polyorganosiloxanes in defoamer preparations. Thus, for example, the use of branched siloxane defoamers in combination with polyether-polysiloxane copolymers as defoamers is recommended, for example, for pulp production (EP-A 341 952). In combination with mineral oils as a carrier oil, polyether-polysiloxane copolymers are said to have a positive effect (U.S. Pat. No. 5,523,019). According to WO 98/000216, siloxanes having dimethyl-3-hydroxypropylpolyoxyethylenepolyoxypropylene groups are particularly suitable as surfactants in defoamer formulations.
EP-A 663 225 and EP-A 1076073 claim crosslinked or branched polyorganosiloxanes which carry at least one polyether moiety, as one of the two components of a defoamer formulation. The crosslinking in this case is via alkylene groups, via polydimethylsiloxanes or via polyether groups. As a result of the linkage via Si—C bonds, the products are stable to hydrolysis. These products, however, are generally of high viscosity and are difficult to handle or to further process. In addition, their deposition behavior is high in the operations in which defoaming is to take place.
The process proposed in U.S. Pat. No. 5,625,024 results in linkages via Si—O—C groups, which are unstable to hydrolysis, particularly in acidic or basic media, and hence lose their activity in a foaming aqueous medium.
DE 102 55 649 A describes defoamer formulations in which specially branched polyether-polysiloxane copolymers are added to known antifoams. The effect of such defoamer formulations, however, is attenuated in the presence of water.
DE 31 33 869 C1 (corresponding to U.S. Pat. No. 4,417,068 A) provides a general description of the preparation of silicone polyethers from hydrosiloxanes and alkenyl polyethers in a noble-metal catalyzed hydrosilylation reaction. The examples show the effect of amount of catalyst and catalyst structure on the conversion of the SiH groups. The SiH conversion obtainable in one time interval can be substantially increased, with a given catalyst, only by means of considerably higher concentrations of noble metal, but this is expensive and hence disadvantageous. Also described is how the preparation uses one alkenyl polyether and one hydrosiloxane. Other silicone polyethers require different raw materials.
U.S. Pat. No. 5,271,868 describes antifoam compositions which are composed predominantly of silicone polyethers. The silicone polyethers consist of a linear siloxane backbone having two different kinds of polyether substituents, and also contain 1 to 10 alkylmethylsiloxy units per molecule, where the alkyl chains have 3 to 20 C atoms. The two types of polyether differ in that one type of polyether contains a molar fraction of 10%-30% ethyleneoxy units, while the other type of polyether has a molar fraction of 65%-85% ethyleneoxy units, the remainder in each case being made up of propyleneoxy units. The different polyether substituents are therefore also different in polarity. Silicone polyethers which, in addition to mixed substituents comprising ethyleneoxy and propyleneoxy units, also include substituents composed of only one kind of alkyleneoxy units are not described. The silicone polyethers are prepared by customary hydrosilylation from an H-siloxane and, in this case, two different unsaturated polyethers, the incorporation of the alkyl chains taking place through further addition of olefins to the H-siloxane, either simultaneously or in a separate synthesis step. The fairly narrow definitional range for these polyethers suggests that due to the different polarity of the polyethers, their mutual miscibility is limited as well, and worsens as the differences between them increase. This effect is not conducive to chemical reaction. The additional incorporation of alkyl groups complicates the preparation process further, since the hydrosilylation characteristics of olefins are different from those of allyl polyethers. According to JP 58-58126, which is cited in U.S. Pat. No. 5,271,868, however, such incorporation is necessary in order to provide further improvement in the performance capacity of antifoams.